Foam and process for producing foam using a carbon dioxide blowing agent

ABSTRACT

The present invention provides a process for producing thermoplastic foam, including: 
     (a) melting a thermoplastic polymer to produce a polymer melt; 
     (b) introducing a carbon dioxide blowing agent into the polymer melt; 
     (c) adding to the polymer melt one or more additives selected from the group consisting of 
     (1) polysiloxane, and 
     (2) mineral oil; and 
     (d) extruding and foaming the melted polymer melt, blowing agent, and one or more additives to produce thermoplastic foam. 
     The resultant thermoplastic foam can be thermoformed into various foamed articles, such as foam packaging trays. The quality of foams made from a carbon dioxide blowing agent, and particularly 100% carbon dioxide, has been found to be substantially improved by adding one or more of the foregoing additives, i.e., polysiloxane, mineral oil and, optionally, polyolefin to the polymer melt during the extrusion and foaming process. Such foams have far less ruptured cells, surface cracks and corrugation than comparable foams made with a carbon dioxide blowing agent but which lack the additives of the present invention.

BACKGROUND OF THE INVENTION

The present invention relates to thermoplastic foams and a process forproducing thermoplastic foams using a carbon dioxide blowing agent. Moreparticularly, the invention relates to a process for producing foamusing a carbon dioxide blowing agent whereby certain additives are usedto produce foams of improved quality.

Thermoplastic foams are generally produced in a tandem extrusion process(two extruders in series). The first extruder melts thermoplasticpolymer to produce a polymer melt. A high-pressure metering pumpdelivers a blowing agent to the polymer melt near the end of the firstextruder where mixing is initiated prior to entering the second extruderwhere further mixing and cooling of the blowing agent and polymer meltoccurs. After exiting the second extruder, the polymer melt passesthrough and becomes a foam structure at a die, generally an annular die.The foam structure, generally in the shape of an annular tube, is thenstretched over a forming mandrel. The annular tube coming off themandrel is slit and opened to form a sheet which is then gathered on oneor more rolls. The rolled foam sheet is typically aged for apredetermined period of time and then thermoformed into a foamedarticle, e.g., a packaging tray.

Blowing agents which are most commonly used at present include aliphatichydrocarbons, e.g., C₃ -C₆ alkanes, and fully or partially halogenatedhydrocarbons, e.g., chlorinated and/or fluorinated hydrocarbons. Suchconventional blowing agents possess or cause one or more of thefollowing undesirable characteristics: pollution, potential damage tothe ozone layer, flammability, poor thermoforming characteristics,brittleness, high cost, and the need for a prolonged aging period toallow some or all of the blowing agent to diffuse from the foam sheetprior to thermoforming. For these reasons, alternatives to the foregoingblowing agents have been sought.

One such alternative is carbon dioxide, which avoids most or all of theforegoing undesirable characteristics of conventional blowing agents.However, due to the extreme volatility and high vapor pressure of carbondioxide, it is a difficult blowing agent to use, often resulting in afoam sheet with ruptured cells, surface defects (e.g., cracks), andcorrugation, i.e., thickness variations within the foam sheet whichproduce a rippled or wavy surface appearance. Each of the foregoingoccurrences detrimentally affect the physical properties of the foam(e.g., strength) as well as the aesthetic qualities thereof.

Accordingly, there is a need in the art for an improved process forproducing thermoplastic foam from a carbon dioxide blowing agent whichproduces a high-quality foam, i.e., one with fewer ruptured cells,surface defects, and corrugation.

SUMMARY OF THE INVENTION

That need is met by the present invention which provides a process forproducing thermoplastic foam, comprising:

(a) melting a thermoplastic polymer to produce a polymer melt;

(b) introducing a blowing agent comprising carbon dioxide into thepolymer melt;

(c) adding to the polymer melt one or more additives selected from thegroup consisting of

(1) polysiloxane, and

(2) mineral oil; and

(d) extruding and foaming the polymer melt, blowing agent, and one ormore additives to produce thermoplastic foam.

Preferably, the blowing agent consists essentially of carbon dioxide,i.e., 100% carbon dioxide. It is also preferred that both polysiloxaneand mineral oil are added to the polymer melt. A polyolefin may also beadded to the polymer melt as an additional additive.

In accordance with another aspect of the present invention, athermoplastic foam is provided which comprises:

(a) a polymeric matrix comprising a thermoplastic polymer and one ormore materials selected from the group consisting of

(1) polysiloxane, and

(2) mineral oil; and

(b) a plurality of cells dispersed throughout the polymeric matrix andcontaining a blowing agent comprising carbon dioxide, preferably 100%carbon dioxide.

The thermoplastic foam may be formed into a thermoformed article such asa packaging tray or bowl. If desired, an oxygen-barrier film may beadhered to the thermoplastic foam to form a packaging tray or bowl withoxygen-barrier functionality.

The inventors have found that the quality of foams made from a carbondioxide blowing agent, and particularly 100% carbon dioxide, can besubstantially improved by adding one or more of the foregoing additives,i.e., polysiloxane, mineral oil and, optionally, polyolefin, to thepolymer melt during the extrusion and foaming process. Such foams haveless ruptured cells, surface cracks and corrugation than comparablefoams made with a carbon dioxide blowing agent but which lack theadditives of the present invention. The improvements of the presentinvention are particularly significant when 100% carbon dioxide, asopposed to a blend of carbon dioxide and a conventional aliphatic orhalogenated blowing agent, is used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a preferred extrusion and foamingsystem in accordance with the present invention; and

FIG. 2 is a schematic drawing of the thermoplastic foam produced in thesystem of FIG. 1 being cut into two sheets and wound on rolls forsubsequent thermoforming.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIGS. 1 and 2, a preferred extrusion/foaming systemfor carrying out the process of the present invention will be described.A thermoplastic polymer, usually in pellet form, is introduced to firstextruder 10 via hopper 12. The polymer can be any thermoplastic polymercapable of being foamed, such as polystyrene, polypropylene, orpolyethylene terephthalate (e.g., APET, CPET, or PETG). First extruder10 is a screw-type extruder which heats, mixes, and melts thethermoplastic polymer to form a polymer melt. A nucleating agent, suchas citric acid, sodium bicarbonate or talc, is optionally added to firstextruder 10 at hopper 12 along with the polymer.

The polymer melt is moved through first extruder 10, cross-over 16, andsecond extruder 20 in the direction indicated by the arrows in FIG. 1. Acarbon dioxide blowing agent, i.e., a blowing agent which comprisescarbon dioxide, is introduced to the polymer melt in first extruder 10at injection port 14. Preferably, the blowing agent consists essentiallyof carbon dioxide. That is, it is preferred that the blowing agent is100% carbon dioxide, notwithstanding any minor amounts of impurities orother substances which may also be present in the carbon dioxide. 100%carbon dioxide is preferred in that it avoids the undesirable effects ofconventional aliphatic or halogenated hydrocarbon blowing agents asdiscussed above. If desired, however, major or minor amounts of otherblowing agents may be blended with carbon dioxide. Such other blowingagents may include nitrogen or conventional aliphatic or halogenatedhydrocarbons, e.g., difluorethane (known commercially as Freon® 152A).

Preferably, the carbon dioxide blowing agent is present in the polymermelt in an amount ranging from about 0.1 to about 15 weight percent, andmore preferably from about 1 to about 3 weight percent, the weightpercents being based on the total weight of the polymer melt. The carbondioxide blowing agent is preferably transported to injection port 14 bya pumping system which generates a pressure of between about 5,000 toabout 5,500 psi.

The polymer melt is mixed with the carbon dioxide blowing agent in firstextruder 10 and then moves through cross-over 16 and into secondextruder 20 wherein the polymer melt/blowing agent mixture is furthermixed and cooled. Although the exact temperatures and pressures withinthe first and second extruders will vary depending upon, e.g., the typeof polymer, the speed at which the polymer is forced through theextruders, etc., typical pressures within first extruder 10 will rangefrom about 1800 to about 3600 psi while pressures within second extruder20 will range from about 1800 to about 3100 psi. The temperature infirst extruder 10 will usually range from about 250° to about 440° F.while the temperature of second extruder 20 will range from about 150°to about 300° F.

Second extruder 20 forces the polymer melt/blowing agent mixture throughannular die 30. The temperature of annular die 30 typically ranges fromabout 250° to about 350° F. while the pressure therein ranges from about800 to 2000 psi. Upon exiting the die, the temperature and pressure ofthe polymer melt/blowing agent mixture suddenly decreases to ambienttemperature and pressure. The decrease in temperature and pressurecauses the polymer melt to begin to harden into a polymeric matrix. Atthe same time, the solubility of the carbon dioxide blowing agent in thepolymer greatly decreases. As a result of these twosimultaneously-occurring phenomena, the polymer melt/blowing agentmixture is transformed into an annular foam tube 40 by virtue of theexpansion of the carbon dioxide blowing agent into a plurality of cellswhich are dispersed throughout the polymeric matrix.

After exiting die 30, the annular foam tube 40 is stretched over formingand cooling mandrel 50 (shown in phantom). The ratio of the diameter ofthe foam tube 40 at mandrel 50 to the diameter of the tube as it exitsdie 30, commonly referred to as the blow-up ratio, may range from about2:1 to about 10:1, but preferably rages from about 3.5:1 to about 5:1.

As shown most clearly in FIG. 2, as the annular foam tube 40 comes offof mandrel 50, it is cut into two foam sheets 70 and 80 by slitter 60.Pull rolls 72, 74 and 82, 84 lead sheets 70 and 80, respectively, towinding rolls 76 and 86, respectively. From rolls 76 and 86, foam sheets70 and 80 will be thermoformed and cut into foamed articles such as,e.g., packaging trays, produce containers, etc. Advantageously, by usinga 100% carbon dioxide blowing agent, very little or no aging time isrequired to allow the blowing agent to diffuse from the cells beforethermoforming. With conventional aliphatic or halogenated hydrocarbonblowing agents, aging time, e.g., from three to seven days, is requiredbefore the foam sheet can be thermoformed.

In the thermoforming process, a roll of foam sheet is fed through anoven to heat it to near its softening point. The hot sheet is thenforced into molds (e.g., tray molds) by vacuum, air pressure, andmechanical pressure. The molded sheet is then fed through a trim presswhere a punch and die mechanism cuts the individual thermoformedarticles from the sheet.

As noted above, carbon dioxide is a very difficult blowing agent to use,primarily due to its extreme volatility and high vapor pressure (at agiven temperature). When used in commercial tandem extrusion/foamingsystems, such as the one described above, carbon dioxide blowing agentsoften result in foam sheets having ruptured cells as well ascorrugation, surface cracks, and other defects. The inventors have foundthat when one or more additives selected from the group consisting ofpolysiloxane, mineral oil, and optionally polyolefin is added to thepolymer melt during the extrusion process, the foregoing problemsassociated with carbon dioxide blowing agents are eliminated or at leastsubstantially reduced. In this manner, high-quality foams can beproduced from a carbon dioxide blowing agent.

Although the reasons for the improvement in foam quality through the useof the foregoing additives is not fully understood, it is theorized thatthe additives serve to minimize heat build-up in the extruders byreducing the sheer energy of the polymer melt. Sheer energy is caused bythe frictional forces generated as carbon dioxide moves past polymermolecules when the carbon dioxide is being mixed with and dissolved inthe polymer melt. In general, the vapor pressure of the blowing agent isdirectly proportional, and the viscosity of the polymer melt isinversely proportional, to the temperature of the polymer melt. If thevapor pressure of the carbon dioxide blowing agent is too high and theviscosity of the polymer melt is too low, the carbon dioxide will have atendency to rapidly diffuse out of the foam upon exiting the die,thereby leading to ruptured cells, corrugation, and a broken or crackedsurface appearance. Because of the extreme volatility of carbon dioxide,it has a tendency to generate very high vapor pressures even at moderatetemperatures. As a result, the inventors have found that it is difficultto sufficiently cool the polymer melt/blowing agent mixture in secondextruder 20 to prevent the rapid diffusion of the carbon dioxide fromthe foam. However, use of one or more of the foregoing additives inaccordance with the present invention appears to "lubricate" the polymerso that the carbon dioxide can mix with and dissolve in the polymer meltwith less heat-generation due to a decrease in the frictional forces(sheer energy) within the polymer melt. Thus, the viscosity of thepolymer melt and vapor pressure of the blowing agent can be bettercontrolled. In this manner, the carbon dioxide expands into smaller,more uniform cells as the foam tube exits the die without rupturing thecells and producing cracks and/or corrugation on the surface of thefoam. As a result, the foams which are produced in accordance with thepresent invention are of high quality, i.e., they have a high degree ofcell integrity and a smooth, aesthetically-pleasing surface appearance.

Unexpectedly, the additives also allow a thicker, lower density foam tobe produced from a carbon dioxide blowing agent without corrugation. Ithas generally been found that as foams made with carbon dioxide are madeless dense and more thick, the severity of corrugation increases. Theuse of one or more of the above additives in accordance with the presentinvention, however, has been found to prevent this from occurring sothat thick (i.e., 120 to 150 mils) foams can be made with a carbondioxide blowing agent without corrugation.

In one embodiment of the present invention, the additive which is addedto the polymer melt is polysiloxane. A preferred polysiloxane ispolydimethylsiloxane. A particularly preferred polysiloxane is anorganomodified polydimethylsiloxane which is available from UnionCarbide Chemicals and Plastics Company, Inc. under the tradenameUCARSIL™ PA-1.

PA-1 may be obtained in a linear low density polyethylene carrier, alongwith diatomaceous earth and silica, from the Specialty PolymerCorporation, P.O. Box 249, Dunbar, W. Va., under the tradename SPC-D55.SPC-D55 has been found to act as a nucleator (as well as improving thequality of the foam due to its PA-1 content). Thus, additionalnucleating agents, such as citric acid or sodium bicarbonate, are notneeded when SPC-D55 (organomodified polydimethylsiloxane, along withdiatomaceous earth and silica) is used as an additive in the process ofthe present invention.

The polysiloxane may be present in the polymer melt in any effectiveamount which achieves a desired level of foam quality. Preferably, thepolysiloxane is present in the polymer melt at a weight percentage of1.0 or less and, more preferably, ranges from about 2×10⁻⁵ to about 0.01weight percent, based on the weight of the polymer melt.

In another embodiment of the present invention, mineral oil, i.e., aliquid petroleum derivative or extract, is used as an additive in theproduction of foams from a carbon dioxide blowing agent. Any mineral oilmay be used. The mineral oil may be present in the polymer melt in anyeffective amount which achieves a desired level of foam quality, butpreferably ranges from about 0.03 to about 0.5 weight percent, based onthe weight of the polymer melt. More preferably, the mineral oil ispresent at a weight percentage of about 0.2. In addition to improvingthe quality of foams made from a carbon dioxide blowing agent, mineraloil has advantageously been found to improve the flexibility of the foamso that the foam is less brittle.

In a most preferred embodiment, both polysiloxane and mineral oil areadded to the polymer melt within the respective weight percentage rangesgiven above for each additive.

In yet another embodiment of the present invention, one or morepolyolefins are included with the polymer melt along with polysiloxanesand/or mineral oil. The polyolefins may be selected from the groupconsisting of ethylene/alpha-olefin copolymer, polypropylene,propylene/ethylene copolymer, ethylene/vinyl acetate copolymer, andethylene/vinyl alcohol copolymer. The polyolefins may be present in thepolymer melt in any effective amount which achieves a desired level offoam quality, but preferably ranges from about 1 to about 15 weightpercent, based on the weight of the polymer melt. More preferably, thepolyolefin is added at a weight percentage of about 8.5.

Referring again to FIG. 1, the introduction of the additives of thepresent invention to the polymer melt will be described. In general, theadditives may be added to the polymer melt at any convenient point inthe process. More preferably, however, the additives are added to thepolymer melt either concurrently with or prior to the addition of thecarbon dioxide blowing agent at injection port 14.

In the case of polysiloxane, it is preferred that this additive be addedto the polymer melt prior to adding the carbon dioxide blowing agent.This is most conveniently accomplished by adding the polysiloxane tofirst extruder 10 at hopper 12 along with the thermoplastic polymer.This can be done either by mixing the polysiloxane (or the polysiloxanemixture in SPC-D55) with the polymer pellets prior to adding the polymerto the hopper, or by adding the polysiloxane to the hopper concurrentlywith but separately from the polymer.

In the case of mineral oil, it is preferred that this additive be addedto the polymer melt concurrently with the addition of the carbon dioxideblowing agent at injection port 14. To this end, a "tee" fitting (notshown) can be attached to injection port 14 whereby carbon dioxideenters the injection port through one "branch" of the tee while mineraloil enters the injection port through another branch of the tee. Bothcarbon dioxide and mineral oil are preferably transported to injectionport 14 by separate pumping systems which generate pressures of betweenabout 5,000 to 5,500 psi. At such pressures, desired volumes of carbondioxide and mineral oil can be supplied to the polymer melt.

Polyolefin is most conveniently provided to the polymer melt in the formof pellets of "barrier foam tray" scrap which is being recycled andadded to hopper 12, either alone or as a mixture with pellets of"virgin" thermoplastic polymer, for processing into a foam in accordancewith the method of the present invention. A barrier foam tray is a foampackaging tray to which a flexible oxygen-barrier film is adhered,thereby providing a packaging tray with oxygen-barrier functionality.Barrier foam trays are described in, e.g., U.S. Pat. Nos. 4,847,148 and4,935,089, and in copending U.S. Ser. No. 08/326,176, the disclosures ofwhich are incorporated herein by reference.

The oxygen-barrier film typically contains one or more polyolefins,including polyolefin-based copolymers, such as ethylene/alpha-olefincopolymer, polypropylene, propylene/ethylene copolymer, ethylene/vinylacetate copolymer, and ethylene/vinyl alcohol copolymer (a preferredoxygen-barrier material). Preferred oxygen-barrier films for adherenceto a foam packaging tray include a bonding layer (the layer which iscontact with and provides adherence to the foam tray) ofstyrene/butadiene copolymer and/or ethylene/vinyl acetate copolymer; anoxygen-barrier layer of ethylene/vinyl alcohol copolymer, vinylidenechloride and copolymers thereof, acrylonitrile, and/or polyamide; and anouter layer of polyethylene, polypropylene, propylene/ethylenecopolymer, and ethylene/alpha-olefin copolymer, including bothhomogeneous and heterogeneous ethylene/alpha-olefin copolymers, such aslinear low density polyethylene. Other internal layers of, e.g.,polyolefins including olefin copolymers such as linear low densitypolyethylene or ethylene/vinyl acetate copolymer, may be included in theoxygen-barrier film to impart desired processing advantages or end-usecharacteristics.

A particularly preferred oxygen-barrier film for adherence to apolystyrene foam tray includes a bonding layer of styrene/butadienecopolymer, an oxygen-barrier layer of ethylene/vinyl alcohol copolymer,and an outer layer of linear low density polyethylene orstyrene/butadiene copolymer.

In the production of barrier foam trays, foam sheets are produced asdescribed above. A flexible oxygen-barrier film is then adhered to thefoam sheet, generally by laminating the film to the foam sheet underheat and pressure with either corona treatment or with a suitableadhesive. The resultant foam sheet/film composite is then thermoformedinto trays or the like and the trays are cut from the sheet. Theremaining portion of the sheet, i.e., after the trays have been cut andremoved therefrom, becomes scrap which is desirably recycled. Otherrecyclable scrap comes from trays which are rejected for quality-controlreasons and from the extruders as a result of start-ups and size andcolor changes. The scrap is ground and pelletized, thereby creating"reclaim pellets." The reclaim pellets comprise the material from whichthe foam sheet was formed, e.g., polystyrene, as well as the polyolefinsand other materials from which the oxygen-barrier film was made. Thesereclaim pellets are then placed into hopper 12, with or without virginpolymer pellets, and extruded and foamed into a foam sheet whichcontains polyolefins as well as the thermoplastic polymer from which thefoam is primarily formed. The foregoing recycle process is described infurther detail in U.S. Pat. Nos. 5,118,561 and 5,330,596, thedisclosures of which are incorporated herein by reference.

It has been surprisingly and unexpectedly found that foams made fromrecycled barrier foam trays as described immediately above by use of acarbon dioxide blowing agent, and specifically a 100% carbon dioxideblowing agent, are superior to carbon dioxide foams which are made fromvirgin polymer (i.e., with no recycled barrier foam material and,therefore, no polyolefin). That is, the polyolefin which is added to thepolymer melt via the reclaim pellets has been found to eliminate orreduce the incidence of surface cracks, corrugation, and cell rupture.This improvement is particularly pronounced when polysiloxane and/ormineral oil are also present in the polymer melt. If desired, purepolyolefin, i.e., not in the form of reclaim pellets, may be added tothe polymer melt.

Regardless of which of the above additives or combination of additivesis used, the process of the present invention results in a thermoplasticfoam, preferably a foam sheet, which comprises a polymeric matrixcontaining a thermoplastic polymer and one or more materials selectedfrom the group consisting of polysiloxane, mineral oil and, optionally,polyolefin. A plurality of (non-ruptured) cells are uniformly dispersedthroughout the polymeric matrix which, at least immediately afterfoaming, contain a carbon dioxide blowing agent which is preferably 100%carbon dioxide. The cells preferably have a size which ranges from about0.25 to about 0.45 millimeters. However, larger or smaller cells can beformed, depending upon the particular thermoplastic polymer, the blowingagent, the additive, the processing conditions, etc. The thickness ofthe foam sheet can range from about 40 to about 200 mils, but preferablyranges from about from about 100 to about 140 mils (1 mil=0.001 inch).

The foam sheet can be thermoformed into a variety of foamed articlessuch as packaging trays or bowls for, e.g., meat, poultry, produce,cheese, etc. Depending upon the oxygen-sensitivity of the product beingpackaged, it may be desirable that the foamed article have anoxygen-barrier film adhered thereto as described above.

The invention will now be more specifically described in the followingexamples, which are intended to be illustrative only and not limiting inscope.

EXAMPLES

In each of the following examples, foam sheets were made in accordancewith the preferred extrusion/foaming method described above andillustrated in FIGS. 1 and 2. In the examples, "quality" of the foamedsheet product or thermoformed article refers to a qualitative, numericalscale ranging from 1 to 5 wherein 1 indicates poor quality and 5indicates excellent quality

EXAMPLE I (COMPARATIVE) CARBON DIOXIDE BLOWING AGENT WITH NO ADDITIVES

Polymer

a. Composition: Polystyrene

b. Flow rate: 695 pounds per hour (PPH)

Blowing Agent

a. Composition: 100% CO₂

b. Amount: 2.8 weight percent (based on the flow rate of polystyrene)

Additive

None

Nucleating Agents

a. Citric acid--0.7 PPH

b. Sodium bicarbonate--0.49 PPH

Process Conditions

a. Primary extruder

1) Temperature range: 355°-415° F.

2) Pressure: 3600 psi

b. Secondary extruder

1) Temperature range: 180°-270° F.

2) Pressure: 3700 psi

c. Die

1) Temperature: 270° F.

2) Pressure: 2500 psi

Foamed Sheet Product

a. Average cell size: 0.25 mm

b. Average sheet thickness: 100 mils

c. Quality

1) Corrugation: 2

2) Open cells: 1

3) Surface cracks: 2

Thermoforming

Not attempted

EXAMPLE 2

Polymer

a. Composition: Polystyrene

b. Flow rate: 779 pounds per hour (PPH)

Blowing Agent

a. Composition: 100% CO₂

b. Amount: 3.3 weight percent (based on the flow rate of polystyrene)

Additive

a. Composition: Mineral oil

b. Amount: 20-25 PPH

Nucleating Agents

a. Composition: Citric acid and sodium bicarbonate

b. Amount: 0.025 weight percent (based on the flow rate of polystyrene)

Process Conditions

a. Primary extruder

1) Temperature range: 356°-444° F.

2) Pressure: 2923 psi

b. Secondary extruder

1) Temperature range: 150°-270° F.

2) Pressure: 2183 psi

Die

1) Temperature: 260° F.

2) Pressure: 1348 psi

Foamed Sheet Product

a. Average cell size: 0.1225 mm

b. Average sheet thickness: 74 mils

c. Quality

1) Corrugation: 3

2) Open cells: 3

3) Surface cracks: 3

Thermoforming

Not attempted.

EXAMPLE 3

Polymer

a. Composition: Polystyrene

b. Flow rate: 238 pounds per hour (PPH)

Blowing Agent

a. Composition: 100% CO₂

b. Amount: 1.5 weight percent (based on the flow rate of polystyrene)

Additive

a. Composition: organomodified polydimethylsiloxane (UCARSIL™ PA-1 fromUnion Carbide), blended with LLDPE, diatomaceous earth, and silica (fromSpecialty Polymer Corp. under the tradename SPC-D55)

b. Amount: 8×10⁻⁴ weight percent (based on the flow rate of polystyrene)

Nucleating Agents

None

Process Conditions

a. Primary extruder

1) Temperature range: 295°-355° F.

2) Pressure: 3000 psi

b. Secondary extruder

1) Temperature range: 182°-212° F.

2) Pressure: 2900 psi

c. Die

1) Temperature: 310° F.

2) Pressure: 2000 psi

Foamed Sheet Product

a. Average cell size: 0.35-0.41 mm

b. Average sheet thickness: 140 mils

c. Quality

1) Corrugation: 3

2) Open cells: 3

3) Surface cracks: 4

Thermoforming

a. Thermoformed article: Tray

b. Quality: 4

EXAMPLE 4

Polymer

a. Composition: Polystyrene

b. Flow rate: 240 pounds per hour (PPH)

Blowing Agent

a. Composition: 100% CO₂

b. Amount: 1.5 weight percent (based on the flow rate of polystyrene)

Additive--Blend Of:

a. Organomodified polydimethylsiloxane (UCARSIL™ PA-1 from UnionCarbide), blended with LLDPE, diatomaceous earth, and silica (fromSpecialty Polymer Corp. under the tradename SPC-D55); present in polymermelt at 0.0025 weight percent (based on the flow rate of polystyrene)

b. Mineral oil; present in polymer melt at 0.2 weight percent (based onthe flow rate of polystyrene)

Nucleating Agents

None

Process Conditions

a. Primary extruder

1) Temperature range: 297°-349° F.

2) Pressure: 3100 psi

b. Secondary extruder

1) Temperature range: 164°-196° F.

2) Pressure: 2900 psi

c. Die

1) Temperature range: 278°-302° F.

Foamed Sheet Product

a. Average cell size: 0.37-0.47 mm

b. Average sheet thickness: 140 mils

c. Quality

1) Corrugation: 5

2) Open cells: 5

3) Surface cracks: 5

Thermoforming

a. Thermoformed article: Tray

b. Quality: 5

EXAMPLE 5

Polymer

a. Composition: Polystyrene

b. Flow rate: 250 pounds per hour (PPH)

Blowing Agent

a. Composition: 100% CO₂

b. Amount: 2.4 weight percent (based on the flow rate of polystyrene)

Additive--Blend Of:

a. Polyolefin; Present in polymer melt at 15 weight percent (based onthe flow rate of polystyrene)

b. Mineral oil; Present in polymer melt at 0.4 weight percent (based onthe flow rate of polystyrene)

Nucleating Agent

a. Composition: talc

b. Amount: 0.19 weight percent (based on the flow rate of polystyrene)

Process Conditions

a. Primary extruder

1) Temperature range: 250°-348° F.

2) Pressure: 2300 psi

b. Secondary extruder

1) Temperature range: 174°-217° F.

2) Pressure: 2300 psi

c. Die

1) Temperature range: 306°-314° F.

2) Pressure: 1900 psi

Foamed Sheet Product

a. Average sheet thickness: 125 mils

b. Quality

1) Corrugation: 4

2) Surface cracks: 4

Thermoforming

a. Thermoformed article: Tray

b. Quality: 3

The foregoing examples demonstrate that CO₂ foams made with an additivein accordance with the present invention are superior to CO₂ foams madewithout such additives.

While the invention has been described with reference to illustrativeexamples, those skilled in the art will understand that variousmodifications may be made to the invention as described withoutdeparting from the scope of the claims which follow.

What is claimed is:
 1. A process for producing thermoplastic foam,comprising:(a) melting a thermoplastic olefin polymer to produce apolymer melt; (b) introducing a blowing agent comprising carbon dioxideinto said polymer melt; (c) adding polysiloxane to said polymer melt;and (d) extruding and foaming said polymer melt, blowing agent, andpolysiloxane to produce thermoplastic foam.
 2. The process of claim 1,wherein said blowing agent consists essentially of carbon dioxide. 3.The process of claim 1, further including the step of thermoforming saidthermoplastic foam into a foamed article.
 4. The process of claim 1,wherein the thermoplastic foam contains a plurality of cells having asize which ranges from about 0.25 to about 0.45 millimeters.
 5. Theprocess of claim 1, .wherein said blowing agent is present in saidpolymer melt in an amount ranging from about 0.1 to about 15 weightpercent, based on the weight of said polymer melt.
 6. The process ofclaim 1, wherein said thermoplastic polymer is selected from the groupconsisting of polystyrene and polypropylene.
 7. The process of claim 1,wherein said polysiloxane is a polydimethylsiloxane modified with anorganic compound.
 8. The process of claim 1, wherein said polysiloxaneis present in said polymer melt in an amount of no greater than about1.0 weight percent, based on the weight of said polymer melt.
 9. Theprocess of claim 1, wherein said one or more additives comprise bothpolysiloxane and mineral oil.
 10. The process of claim 1, wherein saidmineral oil is present in said polymer melt in an amount ranging fromabout 0.03 to about 0.5 weight percent, based on the weight of saidpolymer melt.
 11. The process of claim 1, further including the additionof polyolefin to said polymer melt.
 12. The process of claim 11, whereinsaid polyolefin is selected from the group consisting ofethylene/alpha-olefin copolymer, polypropylene, propylene/ethylenecopolymer, ethylene/vinyl acetate copolymer, and ethylene/vinyl alcoholcopolymer.
 13. The process of claim 11, wherein said polyolefin ispresent in said polymer melt in an amount ranging from about 1 to about15 weight percent, based on the weight of said polymer melt.
 14. Theprocess of claim 1, wherein said one or more additives are added to saidpolymer melt prior to introducing said blowing agent to said polymermelt.
 15. The process of claim 1, wherein said one or more additives areadded to said polymer melt concurrently with the introduction of saidblowing agent to said polymer melt.
 16. The process of claim 1, furtherincluding adding mineral oil to said polymer melt.